The present invention relates to an optical semiconductor device and an optical waveguide, and more particularly to a technology of coupling optical waveguides mutually without a large optical loss.
Demands for a large capacity optical communication device are increasing with a recent increase in communication traffic. Therefore, miniaturization of optical components effective in increasing the capacity of the device is being studied vigorously at various places. For example, a technology of using a Si wire in order to miniaturize an optical waveguide circuit is being studied. The Si wire optical waveguide confines light within the core more strongly in comparison with a conventional case that Silica glass or polymer is used. Therefore, an allowable minimum curvature radius decreases, and the optical waveguide circuit can be made compact. Meanwhile, since the Si wire optical waveguide has a small light spot size, there is a problem that an optical coupling loss with respect to a waveguide type optical device such as a semiconductor laser increases in comparison with the conventional one.
Under this situation described above, a device having a laser beam efficiently coupled with the Si wire by directly bonding the active layer of a semiconductor laser along the optical axis of the Si wire is being studied (A. W. Fang et al., IEEE Photonic Technology Letters, Vol. 18, No. 10. pp. 1143-1145 (2006)). The structure of the device is described with reference to FIGS. 13A, 13B. FIG. 13A is a perspective view, and FIG. 13B is a y-z cross-sectional view taken along A-A′ of FIG. 13A. As shown in those drawings, this device has a Si substrate 11, a Si oxide film layer 12, a Si core layer 13, a compound semiconductor core layer 14 and a compound semiconductor substrate 15 stacked in this order from bottom to top. Further, a high reflecting coating 61 and a low reflecting coating 62 are respectively applied to both end surfaces which are perpendicular to the z-axis in the same manner as an ordinary Fabry-Perot type semiconductor laser. By configuring as described above, the semiconductor laser can be formed along the z-axis defined as an optical axis and the area between z0 and z1 in the drawing defined as a resonator region 21. In other words, laser oscillation can be obtained by performing current injection or light excitation of the above structure. In this case, a gain is produced from the compound semiconductor core layer 14, and the compound semiconductor core layer 14 becomes an active layer of the semiconductor laser. Furthermore, the Si core layer 13 and the compound semiconductor substrate 15 also play roles of a lower cladding layer and an upper cladding layer of the semiconductor laser, respectively. In this case, Si generally has a refractive index higher than that of the compound semiconductor, so that the center of a light intensity distribution 41 is not within the compound semiconductor core layer 14 but within the Si core layer 13 according to the structure as described above (as shown in FIG. 13B). Thus, this device can obtain high optical coupling with the Si wire optical waveguide. FIG. 14 shows the light intensity distribution 41 on a cross section of an xy plane of FIGS. 13A and 13B. As described above, the center of the light intensity distribution 41 is within the Si core layer 13, so that light is mostly confined within the Si core layer 13. However, since light leaks partially into the compound semiconductor core layer 14, a gain can be obtained and laser oscillation becomes possible. To enter the oscillated laser beam into another functional component, optical fiber or the like, an optical circuit may be formed of the Si core layer 13 to guide waveguided light 31 as shown in FIG. 15. Here, a portion extended from the Si core layer 13 in order to guide light to another functional component, namely a portion not having the compound semiconductor core layer 14 and the compound semiconductor substrate 15, may be exposed to air without forming anything on the Si core layer 13 or may have the Si core layer 13 embedded in a polymer, a Si oxide film layer or the like.